I have rock solid GPS reception (# of sats and low HDOP), calibrated magnetometers (two of them) and hovers at exactly half throttle when in stabilized mode. I have a problem though that I noticed from the first flight and cant find a solution no matter how much I searched on the web.

The first time I switch from stabilized mode to PosHold mode the quad shoots up in the sky. I do not know if it would have stopped at any point cause I switch immediately back to stabilize to regain control and bring it back to safe altitude. The same happens the second time I try to switch PosHold but now the climb lasts much shorter and after a few meters it hovers at steady altitude the way it is supposed to.

I am attaching the log file 2017-08-29%2018-10-22.bin here in hope that someone can diagnose the problem and steer me to the right direction. The phenomenon occurs approx at line 13895 and I have noticed there that while BARO.Alt spikes up the way it supposed to during fast climbing the CTUN.Alt parameter goes down... Is this the reason my quad suddenly shoots up? What does CTUN.Alt mean? Is it a glitch and if so why does it occur only when I switch to PosHold and only the first time?

I have read somewhere that the buzzer can interfere with Pixhawk operation. The buzzer does emit a loud beep when I change flight modes. If that is the case though why does it happen only the first and second time and never again in the same flight?